1. Field of the Invention
The invention pertains to the art of image processing and a digital signal processing method and apparatus therefor and, more particularly, to an apparatus for scaling, i.e., enlarging or reducing, an image.
2. Description of Related Art
In digital image processing, the ability to electronically magnify or reduce an image is almost uniformly required as a feature of conventional imaging devices such as copiers and printers. Selected scaling of the image can be limited to a few predetermined choices, but present market tendencies are to demand virtually unlimited scaling options, particularly in the higher quality copying machines. The hardware circuits that have been designed to implement such scaling have been required to capture and process the pertinent image signals with reduced processing time and with minimal storage requirements, for improved operating efficiency and minimization of hardware costs.
The present digital scaling system is directed to a method and control circuit for implementation thereof for selected enlargement, reduction and anamorphic scaling of an image. For the subject disclosure, xe2x80x9canamorphicxe2x80x9d is intended to mean the ability to scale so that the images are enlarged in one dimension and reduced in another dimension; and can also include enlargement or reduction in one dimension with no scaling in the other dimension.
The description includes references to slow-scan and fast-scan digital image data when discussing the orientation of selected window coordinates and sampled image data signals used by control circuitry. For purposes of clarification, data collected along a fast scan direction is intended to refer to individual pixels located in succession along a raster of image information. On the other hand, data collected in the slow-scan direction refers to data derived from a common raster position across multiple rasters of image information. As an example, fast-scan data would refer to the sequential signals collected along the length of the linear photosensitive array during a single exposure period, and is commonly referred to as a raster of data. Slow-scan data would be used to describe signals captured from a plurality of elements along a linear photosensitive array, as the array was moved relative to a document. Analogously, your eyes move in the fast-scan direction relative to this page as you read along each line; when your eyes move to the next line down, they have moved in the slow-scan direction.
Various scaling techniques are known for manipulating static images which are later output onto such media as the screen of a personal computer or onto printed paper. Practically all techniques calculate the value of a new pixel based on the values of neighboring original pixels. For example, linear interpolation uses a matrix of adjacent pixels where the nearest neighbors in the slow scan and fast scan direction define the matrix employed in the calculation of the new pixel value.
A known problem with regard to prior interpolation scaling systems is, particularly with regard to reduction, that such systems operate in a manner to cause an undesired loss of data. Simply stated, linear interpolation throws away data by discarding the values of intermediate original pixels during the calculation of values for the pixels of the reduced image. The loss of such data is a problem sought to be overcome by higher quality imaging systems.
The present invention contemplates a new and more efficient scaling process, which overcomes the above-referenced problem and others to combine a new pixel window averaging process for reduction scaling, in conjunction with linear interpolation for enlargement scaling, to obtain a higher image quality processing system for a digital scaling system.
In accordance with the present invention, there is provided a method and apparatus for selectively enlarging or reducing an image in a plurality of dimensions. The image is scaled in the first dimension by a first scaling process and is scaled in the second dimension by a second scaling process. If the first scaling process comprises an enlargement of the image, the process is an interpolating process, while if the second scaling process is a reduction of the image, the process comprises a pixel window averaging process. The scaling factor of either the enlargement or the reduction is selectively variable for either dimension. Thus, the invention provides a unique advantage of permitting an operator to select different scaling techniques to facilitate the implementation of the best technique for the desired result.
In accordance with another aspect of the present invention, implementation of the distinct scaling processes is achieved in parallel where either the interpolation process or the pixel window averaging process are active for the scaling of the image. Scan line buffers for storage of image scan lines can be shared but only one of the scaling techniques can be implemented at a time.
In accordance with another alternative embodiment of the invention, the distinct scaling process can be serially implemented for concurrent active scaling operations for the image. In such a serial approach, separate scan line buffers for interpolation and pixel window averaging are respectively required. The separate scan line buffers can be used simultaneously to implement the concurrent scaling operations. In accordance with still another alternative embodiment of the present invention, there is provided a method and apparatus for selectively enlarging or reducing an image in a plurality of dimensions, wherein the reducing process is selected according to an image tag associated with each pixel of the image which defines the type of image content in the area encompassing the pixel. The image is scaled in the first dimension by a first scaling process and the image is scaled in the second dimension by a second scaling process. If the first scaling process comprises an enlargement of the image, the process is an interpolating process, while if the second scaling process is a reduction of the image, the process comprises an interpolating process when the associated image tag defines a line drawing image content or a pixel window averaging process when contone image content is defined by the image tag.
In accordance with yet another aspect of the present invention with respect to anamorphic scaling, implementation of the distinct scaling processes is achieved in parallel, in a two-pass process, where either the interpolation process or the pixel window averaging process output is selected for reducing of the image on a first pass in one direction. Scan line buffers for storage of image scan lines are provided in each parallel path. On a second pass in the remaining direction, only the interpolator process is implemented in order to enlarge the image in the remaining direction.
In accordance with still another alternative embodiment of the invention, a second interpolator enlarging process is implemented, serially connected to the output of a first reducing process comprised of a pixel window averaging process and an interpolator process connected in parallel as described in the previous embodiment. The first reducing process is implemented in the manner described above for reducing of the image wherein a multiplexer selects the preferred process output. In this manner this alternative embodiment provides one-pass anamorphic scaling.
One benefit obtained from the present invention is a scaling system which provides overall higher quality imaging in a digital scaling system by providing a selective scaling processes including a more efficient system for image reduction either in combination with, or alternative to, a linear interpolation scaling process.
Another benefit obtained from the present invention is selective implementation of either a serial or parallel approach for the implementation of a scaling system having both an interpolation technique particularly useful for enlargement of an image, and a pixel window averaging technique for reduction of the image. In the parallel approach sharing of scan line buffers provides for reduced hardware costs, while in the serial approach, simultaneous interpolating processing and pixel window averaging processing minimizes processing time.
Still another benefit obtained from the present invention is the unique advantage of using a preferred scaling technique in each area of an image, facilitating the implementation of the best technique for the desired result.
Yet another benefit obtained from the present invention is the advantage of one-pass anamorphic scaling incorporating a preferred scaling technique in each area of an image.
Other benefits and advantages of the subject method and apparatus will become apparent to those skilled in the art upon a reading and understanding of this specification.